1. Technical Field
The invention concerns digital map overlays, and more particularly a method and system for more efficiently producing digital map overlays with reduced processing overhead.
2. Description of the Related Art
Graphical overlays are a desirable feature that can be displayed in conjunction with a moving digital map. These graphical overlays can display important information that is helpful for a user of the map. For example, Height Above Terrain (HAT) and Threat Intervisibility (TIV) are graphical overlay layers used in digital moving maps. HAT is typically represented by a colored region on a map that indicates terrain that is above the current altitude of the aircraft. This is useful for a pilot to indicate which terrain can safely be traversed at their current height. TIV is also commonly represented by a colored region on a digital map. In the case of TIV, however, the colored region indicates that a threat (Radar, SAM site, etc.) can see a particular location at the aircraft's current altitude.
Typically the colored layers comprising a graphical overlay are electronically draped over the top of another by selectively controlling the appearance of pixels or color fragments. This display technique can create a view of the region for the user that is similar to a satellite image of the area, a paper chart, or slope shaded terrain relief. Although such presentation techniques can be very helpful for a user to visualize important features of a particular area, performing the necessary calculations and transferring the patterns to image memory are very time consuming operations. As the scene memory grows in size, these calculations take more time, and more time is needed to load the images into video memory. Consequently, a user can sometimes be required to wait a considerable period of time while the data is calculated and displayed.
The word “post” is commonly used to refer to a point on an image that has an associated description, such as elevation or color. For example, a color fragment is a kind of post that is described by color. To generate the HAT layer, the digital map application must create an overlay image and parse through the elevation data for the current geographic area. To decide if a post is painted or clear, it must compare each elevation post to the current altitude of the aircraft. If the post is greater that the aircraft elevation, then that post must be painted, otherwise it is left clear. A painted post is simply set to some desired color to indicate the terrain is above the aircraft. When this process is done for the entire scene, it can be displayed as the HAT overlay. This process must be repeated anytime the aircraft changes altitude. Usually a threshold is set so that the aircraft must move some fixed amount before this costly calculation is performed.
As another example, it is helpful to consider the processing necessary to generate the TIV layer, the digital map application must first generate a mask (How-Hi mask) that represents a threat pattern. In other words, a new matrix of elevation values is created using user defined threat characteristics. A suitable algorithm can be used to generate a pattern that represents the altitude at each post that an aircraft must be below in order to remain invisible to all threats. Using this information, a pattern is generated that can be used as a new elevation source data. To generate the actual overlay layer, the application must create a new overlay image and perform the same post-by-post comparison as for the HAT layer, except that the post is painted if the source elevation post is below the aircraft's elevation.
The How-Hi mask described above only needs to be updated when the scene moves (and needs to load new elevation source data) or if the user changes the threat characteristics. However, the overlay layer must be recalculated any time the aircraft changes altitude. The combination of all these calculations can be very processor and memory intensive.
One consequence of the foregoing is that conventional systems require a threshold to be programmed into the system such that the aircraft must change elevation by at least the threshold amount before the HAT and TIV overlays are recalculated. Otherwise, the system would constantly be updating the overlays. If the elevation is changing faster than the calculations could be performed (which is generally the case) then the user would never see the changes. This is a problem because the threshold presents a performance limitation.
A variety of commercial off the shelf (COTS) tools are currently available to simplify and standardize the design and development computer graphics display systems. One such tool is the OpenGL Graphics Interface. OpenGL is a well known 3D graphics API (Application Programmer's Interface) that is intended to provide a simple, direct interface to the fundamental operations of 3D graphics rendering. The Open GL API eases the task of writing graphics programming by eliminating the need to write a distinct graphics driver for each platform on which a particular application is to be run. OpenGL is now considered an industry standard and is supported by nearly all COTS video cards. Another common graphics API is DirectX, which is available from Microsoft Corporation of Redmond, Wash.
While the availability of OpenGL and COTS video cards hold much promise for future reductions in cost and complexity of computer graphics systems, they also present certain problems. For example, COTS video cards may not have as much processing capability or video memory as might otherwise be desirable for performing certain functions, such as the display of overlays on moving digital maps. Accordingly, there is a need to find innovative ways to adapt such existing COTS hardware and API's to perform these tasks while remaining within their performance limitations.